The present invention relates to a color information converting device for use in a color image recorder for processing image density signals each being representative of a particular color-separated image density to produce a record density signal representative of the record density of black, cyan, magenta or yellow.
With a color copier or similar color image recorder, it is a common practice to read spectral density data of red (R), green (G), blue (B) and so forth by a color scanner and then convert them into record density data of cyan (C), magenta (M), yellow (Y), black (Bk) and so forth by a color printer. Various methods and apparatuses have heretofore been proposed for implementing such conversion (e.g. Japanese Patent Laid-Open Publication Nos. 125054/1988, 246979/1988, 116277/1988, 161979/1984 and 13260/1986 and Japanese Patent Application Nos. 135833/1988 and 193695/1988). Among them, Laid-Open Publication No. 125054/1988 discloses a color conversion circuit which switches over constants by using signals which are not related to the values of image density signals. Laid-Open Publication No. 246979/1988 teaches a color converting device having an exclusive conversion circuit for full color, an exclusive conversion circuit for monocolor, and a circuit for selecting one of the outputs of the two conversion circuits. Further, Laid-Open Publication No. 135833/1988 proposes a color conversion circuit of the type generating a constant selecting signal which selects constants each being associated with a particular image density signal in matching relation to the values of the image density signals.
Laid-Open Publication No. 161979/1984 mentioned above discloses a device having a plurality of look-up tables (LUT) and a gamma conversion circuit for switching over the LUTs. In this type of device, an exclusive LUT for undercolors is provided to readily implement the addition of an undercolor. Such a scheme, however, needs an extra memory capacity for accommodating undercolors, resulting in an increase in cost.
Laid-Open Publication No. 116277/1988 also mentioned previously proposes a color converting method which executes so-called masking processing on the record density of only a single coloring material by using an equation: EQU Dx=Kxr.multidot.Dr+Kxg.multidot.Dg+Kxb.multidot.Db
where Kxr, Kxg and Kxb are constants, and x is c, m, y or k.
The approach using the above equation, however, has a drawback that it cannot add an undercolor to an image.
Laid-Open Publication No. 13260/1986 shows and describes a method which selects the minimum values of image density signals (Dr, Dg and Db) representative of color-separated image densities and determines a record density of black by using a gamma conversion circuit. A problem with this method is that since unnecessary components contained in cyan, magenta and yellow coloring materials have to be taken into account, an image cannot be recorded in full black. While gamma conversion processing is usually implemented with an LUT, the recording amount pattern of black cannot be varied unless a plurality of LUTs are used, resulting in a complicated circuit arrangement.
Laid-Open Application Nos. 135833/1988 and 193695/1988 each discloses a method which, in the event of recording an image by using a black coloring material also, begins to output black from zero gray scale density. Hence, tones rendered slightly differ from orginal tones.
A color conversion circuit proposed by Laid-Open Publication No. 125054/1988 cannot select constants in matching relation to image density signals and, therefore, cannot correct the failure of the density addition rule. Hence, it is impossible to attain sufficient color reproducibility in the event of color reproduction.
A color converting device disclosed in Laid-Open Publication No. 246979/1988 needs exclusive color conversion circuits for full color and monocolor and, therefore, it is complicated and expensive.
Laid-Open Application No. 135833/1988 teaches a color converting circuit constructed to select constants in association with the values of image density signals and, therefore, capable of correcting the failure of the density addition rule. However, since this kind of approach needs constants associated with image density signals even in a monocolor mode, product sum processing means corresponding to a greater number of constants has to be used to implement combined full-color and monocolor recording. Such circuitry cannot be realized at low cost.